1. Field of the Invention
The present invention relates to high resolution position measurement systems, and more particularly to sub-millimeter resolution, equivalent time bi-static radar time-of-flight radio location systems operating over a range of less than about 100 feet.
2. Description of Related Art
High-resolution bi-static time-of-flight measurement systems employing equivalent time sampling techniques are not common. U.S. Pat. No. 4,01,459, xe2x80x9cSystems for the Detection of Moving Objects within a Survey Area by Microwave Diffractionxe2x80x9d by Jorgen et al and U.S. Pat. No. 5,576,626, xe2x80x9cNarrow Field Electromagnetic Sensor System and Methodxe2x80x9d by McEwan both show bi-static RF beam interruption systems that do not provide time-of-flight measurement. U.S. Pat. No. 5,589,838, xe2x80x9cShort Range Radio Locator Systemxe2x80x9d by McEwan and U.S. Pat. No. 6,054,950, xe2x80x9cUltra Wideband Precision Geolocation Systemxe2x80x9d by Fontana relate to un-tethered transmitters or receivers operating on a time-of-arrival basis requiring N+1 positions to triangulate an N-dimensional fix. In contrast, time-of-flight techniques require only N positions for an N-dimensional fix.
U.S. Pat. Nos. 5,510,800 and 5,661,490, xe2x80x9cTime-of-Flight Radio Location Systemxe2x80x9d by McEwan are relevant prior art. These patents disclose a pulsed transmitter combined with a sampling receiver that samples with swept timing to form an equivalent time replica of a received RF burst, which is processed to indicate the time-of-flight from a transmitter. The transmitter and receiver operate with controlled timing provided by a hard-wire. The limitation in these systems has to do with timing architecture: a reset pulse initiates the start of time-of-flight timing and the receipt of a time-of-flight RF pulse ends of timing. Unfortunately, the reset pulse is not tightly related to the RF time-of-flight and offers no solid means or method to track out delay path variations and compensate inevitable timing skews on the sub-nanosecond level. The prior art does not disclose a means or method to effectively compensate timing skews.
The present invention is a bi-static radar time-of-flight measurement system using differentially-configured sampling receivers to cancel timing skews. A transmitter transmits RF pulses in response to a transmit timing signal. A first sampling receiver samples the transmitted pulses in response to a receive timing signal to produce an equivalent time replica of received RF pulses. A second sampling receiver samples the transmit timing signal to form an equivalent time timing reference, which indicates the initiation of the transmitted RF pulse. This timing reference starts a time-of-flight counter. A detected time-of-flight pulse from the first receiver terminates the time-of-flight counter. The counter thereby indicates the time-of-flight from the first receiver minus systematic delays detected by the second receiver. The two receivers operate differentially with regard to timing delays.
The transmit and receive timing signals can be provided by several techniques including: (1) a swept delay provided by a delay locked loop (DLL) or (2) a phase-slipped delay provided by two precision but slightly frequency-offset oscillators.
A primary object of the present invention is to provide a high accuracy time-of-flight range measurement system.
Applications include low cost rangefinder radars for robotic and industrial controls, fluid level sensing radars, and through tank fill-level measurements.